Anti-reflective coatings for glass surfaces, employing destructive interference of the reflection from thin, transparent films are known from H. A. MacLeod, “Thin Film Optical Filters”, 3rd edition, (2001) IOP Publishing, Bristol and Philadelphia. In essence, the anti-reflective effect is achieved by stacking alternating layers of high- and low-refractive materials of a suitable thickness (where here “refraction” means the optical index of refraction at the selected wave length that can also be complex valued (E. Hecht, A. Zajac, “Optics”, Addison-Wesley (1977)).
Until now, the preference has been to use tantalum pentoxide (Ta2O5) as the highly refractive material, while SiO2 is frequently used as the low-refractive material.
Electrostatic chucks of glass or glass ceramic for lithographic applications have a highly reflective metal electrode, typically a thin Cr film that lies from a few 10 μm up to about 500 μm below the surface. The electrode reflects incident light more strongly than the glass layer above it. Structures and properties of the surface are thus “difficult” to recognize visually/measure optically because they are “drowned” in the scattered light.
When illuminated with unpolarized light, the reflectance of the surface of an SiO2 glass body in air is about 4% in the visible range, while the reflectance of an embedded (bonded) metallic chromium film is about 60% and thus far “outshines” the surface.
In order to measure the chuck surface using interferometry (Fizeau), a highly reflective metal film, as thin as possible, for example of silver or chromium, was vaporized or sputtered onto the (glass) surface and removed after the measurement by chemical etching. This method has a number of disadvantages:
(A) The temporary metal coating generates stresses that may result in a distortion or curvature of the surface and falsify any conclusions derived therefrom concerning the planarity or shape of the uncoated surface. This is known specifically from Cr coatings and is very pronounced, but also cannot be completely prevented, in the case of silver.
(B) Every surface can only be coated homogenously to a finite degree. Typically, metal film thicknesses of about 100 nm are required for planarity measurements that, with typical inhomegeneities in film thickness of 5 to 10%, induce errors in measurement of approximately 5 to 10 nm (independently of film stresses).
(C) The coating and the subsequent etching can cause undesirable changes in the original surface. The micro-roughness in particular of the glass surface can increase as the result of etching processes.
(D) The expenditure in equipment and time in producing a precision chuck surface is seriously hampered by the need for a suitable metallizing process (and a corresponding etching process afterwards) before each interferometry measurement. The result is a substantial increase in cost.
Consequently, it is desirable to take a direct interferometry measurement by furnishing the embedded metal film with an anti-reflective coating on its upper side without metallizing the surface beforehand. The anti-reflective effect must ensure, at least for a narrow range around a selected wavelength (measurement wave length of the interferometer being used) and for the range of almost perpendicular incident light, that only a negligibly small amount of light is reflected from the electrode (negligible relative to the reflection at the glass surface of approximately 4%).